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The present invention relates generally to devices and methods for driving capacitive loads, and more specifically to a power efficient capacitive load driving device that can be used to drive one or more acoustic transducers of a parametric audio system.
Parametric audio systems are known that employ one or more acoustic transducers to project an ultrasonic carrier signal modulated with an audio signal through the air for subsequent regeneration of the audio signal. U.S. patent application Ser. No. 09/758,606 filed Jan. 11, 2001 entitled PARAMETRIC AUDIO SYSTEM discloses a parametric audio system that includes a modulator configured to modulate an ultrasonic carrier signal with an audio signal, a driver amplifier configured to amplify the modulated ultrasonic signal, and one or more acoustic transducers configured to receive the amplified ultrasonic signal and project it through the air along a selected path. Because of the non-linear propagation characteristics of the air, the modulated ultrasonic carrier signal is demodulated as it passes through the air, thereby regenerating the audio signal along the selected path of projection.
Each acoustic transducer included in the above-referenced parametric audio system is a wide-bandwidth capacitive transducer such as a membrane-type electrostatic transducer. Further, the driver amplifier of the parametric audio system includes one or more inductors that can be coupled to the capacitive load of the acoustic transducer, thereby effectively forming a resonant circuit to facilitate the transfer of energy between the driver amplifier and the acoustic transducer.
One drawback of the above-referenced parametric audio system is that the transfer of energy between the driver amplifier and the acoustic transducer can be inefficient and can cause high system power requirements. For example, because the load provided by the acoustic transducer is reactive, a significant amount of the energy delivered to the acoustic transducer by the driver amplifier is reflected back to the driver amplifier, which typically recovers at least a portion of the reflected energy via the above-mentioned inductor and dissipates the remaining energy as heat. Such energy dissipation can increase both the power and cooling requirements of the system.
Another drawback of the above-referenced parametric audio system is that, in some configurations, the driver amplifier may deliver energy to the acoustic transducer with some distortion and/or reduced bandwidth. To address this problem, the inductor of the driver amplifier can be connected to the capacitive load of the acoustic transducer via a damping resistor to dampen the resonance between the inductor and the capacitive load. However, as energy is transferred between the driver amplifier and the acoustic transducer through the damping resistor, at least some of this energy is typically absorbed by the damping resistor, which dissipates the absorbed energy as heat to further degrade the power efficiency of the system.
It would therefore be desirable to have a more power efficient capacitive load driving device. Such a device would be configurable to drive one or more acoustic transducers of a parametric audio system. It would also be desirable to have a capacitive load driving device that can be used to drive an acoustic transducer of a parametric audio system with low distortion and to assure wideband output capability.
In accordance with the present invention, a device and method for driving a capacitive load is provided that has a more power efficient design. The power efficient capacitive load driving device can be used to drive one or more acoustic transducers of a parametric audio system with low distortion and increased bandwidth. Such benefits are achieved by coupling the capacitive load driving device to a capacitive load, and driving the capacitive load with at least one controlled switched drive signal.
In one embodiment, the capacitive load driving device includes a current source, a plurality of switches interconnected in an xe2x80x9cH-bridgexe2x80x9d configuration coupled to an output of the current source, and a controller. The current source comprises a DC current source including a voltage source having a negative terminal connected to ground potential and a positive terminal, an optional charge capacitor coupled between the positive terminal of the voltage source and the ground potential, and a relatively large-valued inductor having first and second terminals. The relatively large inductor value is selected to achieve a desired low resonant frequency value with the capacitive load. The DC current source further includes a first switch connected between the positive terminal of the voltage source and the first terminal of the inductor and configured to allow current to flow from the voltage source through the first switch to the inductor, and a diode connected between the first terminal of the inductor and the ground potential and configured to allow current to flow from the ground potential through the diode to the inductor. The second terminal of the inductor comprises the DC current source output, which provides the drive energy to the capacitive load.
The plurality of switches interconnected in the xe2x80x9cH-bridgexe2x80x9d configuration includes a first pair of series-connected switches and a second pair of series-connected switches. Each of the first and second pairs of series-connected switches is connected between the output of the DC current source and the ground potential. Further, the capacitive load is coupled between the node connection of the first pair of series-connected switches and the corresponding node connection of the second pair of series-connected switches. The first switch, which is connected between the voltage source and the inductor, and the plurality of switches interconnected in the H-bridge configuration, include respective control terminals.
The controller is configured to (1) receive a first input signal representative of a predetermined input waveform, a second input signal representative of a measured voltage level across the capacitive load, and a third input signal representative of a measured current level through the inductor, and (2) control the operation of the DC current source and the plurality of interconnected switches to generate an output voltage waveform across the capacitive load that corresponds to the predetermined input waveform. In a preferred embodiment, the controller employs a mathematically optimal control algorithm to control the operation of the DC current source and the plurality of interconnected switches. Such control is carried out by generating suitable control signals and applying the control signals to the respective control terminals of the switches.
The following control activities are presented for purposes of illustration. The controller may perform these control activities at fixed or variable time intervals. In the event it is determined that (1) the level of the predetermined input waveform is increasing and (2) the capacitive load voltage level is less than the level of the predetermined input voltage, the controller controls the DC current source and the plurality of interconnected switches to provide a first controlled switched drive signal to the capacitive load that causes the capacitive load to charge positively. In the event it is determined that (1) the level of the predetermined input waveform is increasing and (2) the capacitive load voltage level is greater than or equal to the level of the predetermined input voltage, the controller controls the DC current source and the plurality of interconnected switches to hold the charge on the capacitive load.
In the event it is determined that (1) the level of the predetermined input waveform is decreasing and (2) the capacitive load voltage level is greater than the level of the predetermined input voltage, the controller controls the DC current source and the plurality of interconnected switches to provide a second controlled switched drive signal to the capacitive load that causes the capacitive load to discharge or charge negatively. In the event it is determined that (1) the level of the predetermined input waveform is decreasing and (2) the capacitive load voltage level is less than or equal to the level of the predetermined input voltage, the controller controls the DC current source and the plurality of interconnected switches to hold the charge on the capacitive load. In this way, the controller controls the operation of the DC current source and the plurality of interconnected switches to generate the output voltage waveform across the capacitive load corresponding to the predetermined input waveform.
By driving a capacitive load with at least one controlled switched drive signal, the presently disclosed capacitive load driving device delivers (recovers) energy to (from) the capacitive load in a more efficient manner, thereby generating a desired output voltage waveform across the capacitive load with increased power efficiency. Further, because the presently disclosed device includes reactive elements (e.g., the current source inductor) that store energy rather than merely dissipate energy as heat, system power and cooling requirements are reduced.
Other features, functions, and aspects of the invention will be evident from the Detailed Description of the Invention that follows.